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Controlled Morphology of Electrochemically Deposited CuSCN by Variation of Applied Bias Voltage
Available online at www.sciencedirect.com
ScienceDirect Energy Procedia 54 (2014) 777 – 781
4th International Conference on Advances in Energy Research 2013, ICAER 2013
Controlled morphology of electrochemically deposited CuSCN by variation of applied bias voltage Soham Ghosha and Shaibal K Sarkara* a
Department of Energy Science and Engineering, Indian Institute of Technology Bombay, Mumbai, India
Abstract CuSCN is a p-type semiconductor essentially used in semiconductor sensitized solar cells for scavenging the photogenerated holes. Here we report, cathodic electrodeposition of CuSCN using NaSCN and Cu2+. Control over the film morphology is found to be highly dependent on the applied potential. One-dimensional rod like morphology with a strong (003) orientation is obtained under lower applied potential. Under higher applied potential the morphology changes to a more granular type. Films are structurally characterized by x-ray diffraction (XRD), scanning electron microscope (SEM) and transmission electron microscopy (TEM) with small angle electron diffraction (SAED). © 2014 Shaibal K. Sarkar. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license © 2014 The Authors. Published by Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/3.0/). Selection and peer-review under responsibility of Organizing Committee of ICAER 2013. Selection and peer-review under responsibility of Organizing Committee of ICAER 2013
Keywords:CuSCN; Nano-rods; Potentiostatic electrodeposition.
1. Introduction Application of earth abundant materials in photovoltaic devices is indeed important for sustainability. Till date, not much effort was put forward on understanding p-type materials as opposed to the n-type materials. Wide bandgap p-type materials like CuI and CuSCN pose interesting device compatibility with good hole mobility[1]. Among the above, CuSCN is widely used as a hole conducting medium in sensitized solar cells[2, 3]. Being a wide band gap semiconductor, CuSCN with a bandgap of 3.7 eV, can be very favorable to use as a window layer in bulk heterojunction solar cells. CuSCN can be deposited by various methods, such as drop-casting[2, 3], spin coating[4], electro-chemical deposition[5-8] SILAR method[9], alumina template process[10] etc. Among these only a few reports on nanostructured CuSCN as a building block material can be found. __________ * Corresponding author. Tel.: +91-22-2576-7846; fax: +91-22-2576-4890. E-mail address: [email protected]
1876-6102 © 2014 Shaibal K. Sarkar. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/). Selection and peer-review under responsibility of Organizing Committee of ICAER 2013 doi:10.1016/j.egypro.2014.07.320
778
Soham Ghosh and Shaibal K. Sarkar / Energy Procedia 54 (2014) 777 – 781
In this report we present new cathodic electrodeposition of CuSCN using NaSCN under varied potential from a unique deposition solution using a combination of water and ethanol. Here we show an applied potential dependent morphology that can be tuned from a 3D nanostructured morphology to a quasi-2D film formation. The as deposited films are optically uniform throughout the substrate. Structural characterizations are performed using electron microscopy, x-ray diffraction etc. 2. Experimental FTO (fluorine doped tin oxide) coated glasses (8 Ω/□) were cleaned with soap solution and then subsequently in piranha solution (a 3:1 mixture of concentrated sulphuric acid and hydrogen peroxide) for 15 minutes and finally rinsed with DI water. Before any deposition, substrates were treated in a 45% HNO3 solution and rinsed with DI water. Films were deposited from a solution containing 0.01M Cu(ClO4)2 (Alfa Aesar), 0.1M NaClO4(Merck), and 0.005M NaSCN (Merck) dissolved in water and ethanol solvent (3:1). Potentiostatic electrochemical depositions were carried out using Autolab PGSTAT302N with Pt and Ag/AgCl as counter and reference electrode respectively. Film morphology was studied with Jeol JSM-7600F Field Emission Gun-Scanning Electron Microscopes (FEG-SEM). TEM was carried out using Jeol 2100F instrument operated under 200kV. For TEM measurements samples were scratched by a razor blade and dispersed in IPA that was subsequently transferred onto the TEM grid and dried naturally. X-Ray Diffraction (XRD) patterns were obtained by using Phillips X'Pert Diffractometer under θ-2θ configuration using Cu-KDradiation. 3. Results and discussion CuSCN electrodeposition was studied with a motivation to use it as a building block of bulk heterojunction solar cells. The electrodeposition of CuSCN on FTO glass was carried out from a solution containing Cu2+ and SCN- ions. An unique solvent consisting of water and ethanol was used. The electrodeposition process was under diffusion limited condition and hence no stirring of the solution was done. Mechanistically, the deposition occurred via the following intermediate reactions under aqueous condition: Cu(ClO4)2 NaSCN Cu+2 + SCNCuSCN+ + e-
Æ Æ Æ Æ
Cu2+ + 2ClO4Na+ + SCNCuSCN+ CuSCN
To find the reduction potential of the electrolyte cyclic voltamogram was studied on FTO coated glass substrate. A broad reduction peak was observed with respect to Ag/AgCl electrode around -0.3V and was asymmetrically stretched till -0.42V. For the present study this potential range was chosen for film deposition. The structural XRD analysis confirmed CuSCN phase of all the films. Figure 1 shows the XRD patterns of films deposited on FTO glass substrates at deposition potential varied from -0.3V to -0.42V. All the patterns Figure1: XRD pattern of CuSCN deposited at varying potential. were matched with single standard JCPDS data (00-0290581) which exhibits crystalline rhombohedral βCuSCN. The pattern collectively showed the deposited material has a strong orientation in <003> direction under all applied potential within the stipulated range. A clear crystallographic change that indeed contribute to the overall morphology was found by the evolution of (101) reflection peak. Under higher applied potential, a considerable
Soham Ghosh and Shaibal K. Sarkar / Energy Procedia 54 (2014) 777 – 781
fractional increase in the (101) peak intensity was found. Along with the (101) reflection peak, similar changes in the (006) and (015) peak intensity was observed that increased with the applied bias voltage. This clearly indicates a possible change in film morphology. Furthermore, morphology study of the nanorods were performed with SEM analysis. Figure 2 shows the SEM images of the different CuSCN films on FTO glass substrates deposited under varied applied potentials. SEM analysis reveals the chronological agglomeration of CuSCN nanorods with increase in potential. Under lower potential, -0.3V vs. Ag/AgCl reference electrode, quasi one dimensional nanorod type morphology was obtained (Figure 2A). The nanorods were found vertically aligned with respect to the substrate. The average diameter of the rods is 140nm. Increase in the applied potential (-0.31V) CuSCN forms a flowery structure that slowly gets agglomerated with further increase in potential above -0.32V as depicted clearly in Figure 2B and 2C respectively. Similarly, with further increase in applied potential, the morphology of the film tends to change (Fig. 2D & 2E) and at -0.42V(Fig. 2F), the film morphology becomes completely different than the first one. We believe that the subsequent change in the morphology is initially governed by the nucleation. Under relatively higher potential, higher nucleation density results in low porosity. As a result, under very high applied potential (-0.4V and above) a complete morphological change is observed.
1 μm
1 μm
1 μm
1 μm
1 μm
1 μm
Figure 2 SEM images of the CuSCN films deposited at (A) -0.30 V, (B) -0.31 V, (C) -0.32 V, (D) -0.35 V, (E) -0.40 V (F) -0.42 V
As for the formation of well oriented CuSCN nanorods, -0.3 V was found to be the most favourable condition.
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Soham Ghosh and Shaibal K. Sarkar / Energy Procedia 54 (2014) 777 – 781
Therefore, keeping the potential constant, time dependent deposition was also analysed. It is seen that the diameter of the nanorods are dependent on the deposition time. SEM images in Figure 3 depict the time dependent study of the films deposited for (A) 1hour, (B) 2 hours and (C) 3 hours respectively. The increase in the rod diameter can be well attributed to the relatively much lower surface reactivity. A similar observation was found for ZnO and MoO 3 nanorod deposition as reported earlier. Thus for further applications we kept the deposition constant at -0.3V and for 1hr. 101 003 113
1 μm
(101) (101 (10 01) 01
1 μm
(0 (003 ((003) 0003 03) 3) 3)
1 μm
Figure 3 SEM of time dependant deposition film for (A) 1hr (B) 2hr (C) 3hr
Figure 4 Transmission electron microscopy of CuSCN nanorods
The microstructural properties of CuSCN nanorods have been studied by transmission electron microscopy (TEM).The micrograph reveals long ordered nanorod shape of CuSCN with diameter of 140 nm as shown in Figure 4. The diffraction pattern obtained from TEM is in good agreement with the XRD analysis and confirms the crystalline nature of nanorods. The d-spacing for (003), (101) and (113) planes obtained from the selected area diffraction pattern (SAED) are found to be 5.48, 3.273 and 1.819 Angstrom respectively. The aforementioned dspacing values are well matched with the JCPDS data card 00-029-0581 which again validates the XRD patterns. 3. Conclusion CuSCN was successfully electrodeposited from a water-ethanol mixture containing Cu+2 and SCN- ions. The morphology of the deposited thin films was tuned from nanorod structure to granular ones by variation of the applied potential in the range of -0.3V to -0.42V. Crystalline rhombohedral β-CuSCN phase was attained for all the films electrodeposited in this potential range that was further verified from TEM measurements. Strong orientation in <003> direction was obtained under -0.3V bias voltage and 1 hour deposition time. Nanorods having an average diameter of 140nm were achieved. Acknowledgements Authors thank Defence Research and Development Organisation for financial support. SG thanks Neha Chaki Roy for her help in analytical measurements. References [1] Rost C, Sieber I, Fischer C, Lux-Steiner MC, Könenkamp R, Semiconductor growth on porous substrates, Materials Science and Engineering: B, (2000); 69–70:570. [2] O'Regan B, Lenzmann F, Muis R, Wienke J, A Solid-State Dye-Sensitized Solar Cell Fabricated with Pressure-Treated P25−TiO2 and CuSCN: Analysis of Pore Filling and IV Characteristics, Chemistry of Materials, (2002); 14:5023.
Soham Ghosh and Shaibal K. Sarkar / Energy Procedia 54 (2014) 777 – 781 [3] Kumara GRRA, Konno A, Senadeera GKR, Jayaweera PVV, De Silva DBR., Tennakone K, Dye- sensitized solar cell with the hole collector p-CuSCN deposited from a solution in n-propyl sulphide, Solar Energy Materials and Solar Cells, (2001); 69:195. [4] Tennakone K, Kumara GRRA, Kottegoda IRM, Perera VPS, Weerasundara PSRS, Sensitization of nano-porous films of TiO2 with santalin (red sandalwood pigment) and construction of dye-sensitized solid-state photovoltaic cells, Journal of Photochemistry and Photobiology A: Chemistry, (1998); 117:137. [5] Wu W, Jin Z, Hua Z, Fu Y, Qiu J, Growth mechanisms of CuSCN films electrodeposited on ITO in EDTA- chelated copper(II) and KSCN aqueous solution, Electrochimica Acta, (2005); 50:2343. [6] Wu WB, Jin ZG, Hu GD, Bu SJ, Electrochemical deposition of p-type CuSCN in porous n-type TiO2 films, Electrochimica Acta, (2007); 52:4804. [7] Ni Y, Jin Z, Fu Y, Electrodeposition of p-Type CuSCN Thin Films by a New Aqueous Electrolyte With Triethanolamine Chelation, Journal of the American Ceramic Society, (2007); 90:2966. [8] Selk Y, Yoshida T, Oekermann T, Variation of the morphology of electrodeposited copper thiocyanate films, Thin Solid Films, (2008); 516:7120. [9] Sankapal BR, Goncalves E, Ennaoui A, Lux-Steiner MC, Wide band gap p-type windows by CBD and SILAR methods, Thin Solid Films, (2004); 451–452:128. [10] Son Y, Tacconi NR de, Rajeshwar K, Photoelectrochemistry and Raman spectroelectrochemistry of cuprous thiocyanate films on copper electrodes in acidic media, Journal of Electroanalytical Chemistry, (1993); 345:135.
781
ScienceDirect Energy Procedia 54 (2014) 777 – 781
4th International Conference on Advances in Energy Research 2013, ICAER 2013
Controlled morphology of electrochemically deposited CuSCN by variation of applied bias voltage Soham Ghosha and Shaibal K Sarkara* a
Department of Energy Science and Engineering, Indian Institute of Technology Bombay, Mumbai, India
Abstract CuSCN is a p-type semiconductor essentially used in semiconductor sensitized solar cells for scavenging the photogenerated holes. Here we report, cathodic electrodeposition of CuSCN using NaSCN and Cu2+. Control over the film morphology is found to be highly dependent on the applied potential. One-dimensional rod like morphology with a strong (003) orientation is obtained under lower applied potential. Under higher applied potential the morphology changes to a more granular type. Films are structurally characterized by x-ray diffraction (XRD), scanning electron microscope (SEM) and transmission electron microscopy (TEM) with small angle electron diffraction (SAED). © 2014 Shaibal K. Sarkar. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license © 2014 The Authors. Published by Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/3.0/). Selection and peer-review under responsibility of Organizing Committee of ICAER 2013. Selection and peer-review under responsibility of Organizing Committee of ICAER 2013
Keywords:CuSCN; Nano-rods; Potentiostatic electrodeposition.
1. Introduction Application of earth abundant materials in photovoltaic devices is indeed important for sustainability. Till date, not much effort was put forward on understanding p-type materials as opposed to the n-type materials. Wide bandgap p-type materials like CuI and CuSCN pose interesting device compatibility with good hole mobility[1]. Among the above, CuSCN is widely used as a hole conducting medium in sensitized solar cells[2, 3]. Being a wide band gap semiconductor, CuSCN with a bandgap of 3.7 eV, can be very favorable to use as a window layer in bulk heterojunction solar cells. CuSCN can be deposited by various methods, such as drop-casting[2, 3], spin coating[4], electro-chemical deposition[5-8] SILAR method[9], alumina template process[10] etc. Among these only a few reports on nanostructured CuSCN as a building block material can be found. __________ * Corresponding author. Tel.: +91-22-2576-7846; fax: +91-22-2576-4890. E-mail address: [email protected]
1876-6102 © 2014 Shaibal K. Sarkar. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/). Selection and peer-review under responsibility of Organizing Committee of ICAER 2013 doi:10.1016/j.egypro.2014.07.320
778
Soham Ghosh and Shaibal K. Sarkar / Energy Procedia 54 (2014) 777 – 781
In this report we present new cathodic electrodeposition of CuSCN using NaSCN under varied potential from a unique deposition solution using a combination of water and ethanol. Here we show an applied potential dependent morphology that can be tuned from a 3D nanostructured morphology to a quasi-2D film formation. The as deposited films are optically uniform throughout the substrate. Structural characterizations are performed using electron microscopy, x-ray diffraction etc. 2. Experimental FTO (fluorine doped tin oxide) coated glasses (8 Ω/□) were cleaned with soap solution and then subsequently in piranha solution (a 3:1 mixture of concentrated sulphuric acid and hydrogen peroxide) for 15 minutes and finally rinsed with DI water. Before any deposition, substrates were treated in a 45% HNO3 solution and rinsed with DI water. Films were deposited from a solution containing 0.01M Cu(ClO4)2 (Alfa Aesar), 0.1M NaClO4(Merck), and 0.005M NaSCN (Merck) dissolved in water and ethanol solvent (3:1). Potentiostatic electrochemical depositions were carried out using Autolab PGSTAT302N with Pt and Ag/AgCl as counter and reference electrode respectively. Film morphology was studied with Jeol JSM-7600F Field Emission Gun-Scanning Electron Microscopes (FEG-SEM). TEM was carried out using Jeol 2100F instrument operated under 200kV. For TEM measurements samples were scratched by a razor blade and dispersed in IPA that was subsequently transferred onto the TEM grid and dried naturally. X-Ray Diffraction (XRD) patterns were obtained by using Phillips X'Pert Diffractometer under θ-2θ configuration using Cu-KDradiation. 3. Results and discussion CuSCN electrodeposition was studied with a motivation to use it as a building block of bulk heterojunction solar cells. The electrodeposition of CuSCN on FTO glass was carried out from a solution containing Cu2+ and SCN- ions. An unique solvent consisting of water and ethanol was used. The electrodeposition process was under diffusion limited condition and hence no stirring of the solution was done. Mechanistically, the deposition occurred via the following intermediate reactions under aqueous condition: Cu(ClO4)2 NaSCN Cu+2 + SCNCuSCN+ + e-
Æ Æ Æ Æ
Cu2+ + 2ClO4Na+ + SCNCuSCN+ CuSCN
To find the reduction potential of the electrolyte cyclic voltamogram was studied on FTO coated glass substrate. A broad reduction peak was observed with respect to Ag/AgCl electrode around -0.3V and was asymmetrically stretched till -0.42V. For the present study this potential range was chosen for film deposition. The structural XRD analysis confirmed CuSCN phase of all the films. Figure 1 shows the XRD patterns of films deposited on FTO glass substrates at deposition potential varied from -0.3V to -0.42V. All the patterns Figure1: XRD pattern of CuSCN deposited at varying potential. were matched with single standard JCPDS data (00-0290581) which exhibits crystalline rhombohedral βCuSCN. The pattern collectively showed the deposited material has a strong orientation in <003> direction under all applied potential within the stipulated range. A clear crystallographic change that indeed contribute to the overall morphology was found by the evolution of (101) reflection peak. Under higher applied potential, a considerable
Soham Ghosh and Shaibal K. Sarkar / Energy Procedia 54 (2014) 777 – 781
fractional increase in the (101) peak intensity was found. Along with the (101) reflection peak, similar changes in the (006) and (015) peak intensity was observed that increased with the applied bias voltage. This clearly indicates a possible change in film morphology. Furthermore, morphology study of the nanorods were performed with SEM analysis. Figure 2 shows the SEM images of the different CuSCN films on FTO glass substrates deposited under varied applied potentials. SEM analysis reveals the chronological agglomeration of CuSCN nanorods with increase in potential. Under lower potential, -0.3V vs. Ag/AgCl reference electrode, quasi one dimensional nanorod type morphology was obtained (Figure 2A). The nanorods were found vertically aligned with respect to the substrate. The average diameter of the rods is 140nm. Increase in the applied potential (-0.31V) CuSCN forms a flowery structure that slowly gets agglomerated with further increase in potential above -0.32V as depicted clearly in Figure 2B and 2C respectively. Similarly, with further increase in applied potential, the morphology of the film tends to change (Fig. 2D & 2E) and at -0.42V(Fig. 2F), the film morphology becomes completely different than the first one. We believe that the subsequent change in the morphology is initially governed by the nucleation. Under relatively higher potential, higher nucleation density results in low porosity. As a result, under very high applied potential (-0.4V and above) a complete morphological change is observed.
1 μm
1 μm
1 μm
1 μm
1 μm
1 μm
Figure 2 SEM images of the CuSCN films deposited at (A) -0.30 V, (B) -0.31 V, (C) -0.32 V, (D) -0.35 V, (E) -0.40 V (F) -0.42 V
As for the formation of well oriented CuSCN nanorods, -0.3 V was found to be the most favourable condition.
779
780
Soham Ghosh and Shaibal K. Sarkar / Energy Procedia 54 (2014) 777 – 781
Therefore, keeping the potential constant, time dependent deposition was also analysed. It is seen that the diameter of the nanorods are dependent on the deposition time. SEM images in Figure 3 depict the time dependent study of the films deposited for (A) 1hour, (B) 2 hours and (C) 3 hours respectively. The increase in the rod diameter can be well attributed to the relatively much lower surface reactivity. A similar observation was found for ZnO and MoO 3 nanorod deposition as reported earlier. Thus for further applications we kept the deposition constant at -0.3V and for 1hr. 101 003 113
1 μm
(101) (101 (10 01) 01
1 μm
(0 (003 ((003) 0003 03) 3) 3)
1 μm
Figure 3 SEM of time dependant deposition film for (A) 1hr (B) 2hr (C) 3hr
Figure 4 Transmission electron microscopy of CuSCN nanorods
The microstructural properties of CuSCN nanorods have been studied by transmission electron microscopy (TEM).The micrograph reveals long ordered nanorod shape of CuSCN with diameter of 140 nm as shown in Figure 4. The diffraction pattern obtained from TEM is in good agreement with the XRD analysis and confirms the crystalline nature of nanorods. The d-spacing for (003), (101) and (113) planes obtained from the selected area diffraction pattern (SAED) are found to be 5.48, 3.273 and 1.819 Angstrom respectively. The aforementioned dspacing values are well matched with the JCPDS data card 00-029-0581 which again validates the XRD patterns. 3. Conclusion CuSCN was successfully electrodeposited from a water-ethanol mixture containing Cu+2 and SCN- ions. The morphology of the deposited thin films was tuned from nanorod structure to granular ones by variation of the applied potential in the range of -0.3V to -0.42V. Crystalline rhombohedral β-CuSCN phase was attained for all the films electrodeposited in this potential range that was further verified from TEM measurements. Strong orientation in <003> direction was obtained under -0.3V bias voltage and 1 hour deposition time. Nanorods having an average diameter of 140nm were achieved. Acknowledgements Authors thank Defence Research and Development Organisation for financial support. SG thanks Neha Chaki Roy for her help in analytical measurements. References [1] Rost C, Sieber I, Fischer C, Lux-Steiner MC, Könenkamp R, Semiconductor growth on porous substrates, Materials Science and Engineering: B, (2000); 69–70:570. [2] O'Regan B, Lenzmann F, Muis R, Wienke J, A Solid-State Dye-Sensitized Solar Cell Fabricated with Pressure-Treated P25−TiO2 and CuSCN: Analysis of Pore Filling and IV Characteristics, Chemistry of Materials, (2002); 14:5023.
Soham Ghosh and Shaibal K. Sarkar / Energy Procedia 54 (2014) 777 – 781 [3] Kumara GRRA, Konno A, Senadeera GKR, Jayaweera PVV, De Silva DBR., Tennakone K, Dye- sensitized solar cell with the hole collector p-CuSCN deposited from a solution in n-propyl sulphide, Solar Energy Materials and Solar Cells, (2001); 69:195. [4] Tennakone K, Kumara GRRA, Kottegoda IRM, Perera VPS, Weerasundara PSRS, Sensitization of nano-porous films of TiO2 with santalin (red sandalwood pigment) and construction of dye-sensitized solid-state photovoltaic cells, Journal of Photochemistry and Photobiology A: Chemistry, (1998); 117:137. [5] Wu W, Jin Z, Hua Z, Fu Y, Qiu J, Growth mechanisms of CuSCN films electrodeposited on ITO in EDTA- chelated copper(II) and KSCN aqueous solution, Electrochimica Acta, (2005); 50:2343. [6] Wu WB, Jin ZG, Hu GD, Bu SJ, Electrochemical deposition of p-type CuSCN in porous n-type TiO2 films, Electrochimica Acta, (2007); 52:4804. [7] Ni Y, Jin Z, Fu Y, Electrodeposition of p-Type CuSCN Thin Films by a New Aqueous Electrolyte With Triethanolamine Chelation, Journal of the American Ceramic Society, (2007); 90:2966. [8] Selk Y, Yoshida T, Oekermann T, Variation of the morphology of electrodeposited copper thiocyanate films, Thin Solid Films, (2008); 516:7120. [9] Sankapal BR, Goncalves E, Ennaoui A, Lux-Steiner MC, Wide band gap p-type windows by CBD and SILAR methods, Thin Solid Films, (2004); 451–452:128. [10] Son Y, Tacconi NR de, Rajeshwar K, Photoelectrochemistry and Raman spectroelectrochemistry of cuprous thiocyanate films on copper electrodes in acidic media, Journal of Electroanalytical Chemistry, (1993); 345:135.
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